Conventional gear reducers include an input, a stationary ring gear, a moveable ring gear, and at least one planetary gear, wherein the planetary gear teeth mesh with stationary and moveable ring gear teeth. Such conventional gear reducers may be used to obtain high gear reduction ratios, but since the stationary ring gear and the moveable ring gear have different numbers of teeth to allow for relative movement between the two gears, typically two sets of planetary gears are required, one set of planetary gears to mesh with each of the stationary ring gear and the moveable ring gear.
However, devices have been developed whereby a single set of planetary gears mesh with both the stationary ring gear and the moveable ring gear. In one such arrangement, in which the stationary ring gear and the moveable ring gear have different pitch diameters, the standard addendum dimensions of the ring gears are modified to yield equal internal diameters.